The present invention relates to a projection-type image display apparatus that displays a color image with one light valve serving as a modulating member.
A liquid crystal projector that now is the mainstream in the market of large-screen displays uses a light source lamp, a focusing lens and a projection lens to magnify and form an image of a liquid crystal panel (a light valve) onto a screen. Current commercial systems can be classified roughly into a three-plate system and a single-plate system.
In the former system of the three-plate liquid crystal projector, after a light beam from a white light source is separated into light beams of three primary colors of red, green and blue by a color separation optical system, these light beams are modulated by three monochrome liquid crystal panels so as to form images of the three primary colors. Thereafter, these images are combined by a color combination optical system so as to be projected onto a screen by one projection lens. Since the entire spectrum of the white light from the light source can be utilized, this system has a high efficiency of light utilization. However, because of the necessity of the three liquid crystal panels, the color separation optical system, the color combination optical system and a convergence adjusting mechanism between the liquid crystal panels, this system is relatively expensive.
On the other hand, a conventional single-plate system liquid crystal projector is compact and inexpensive because an image formed on a liquid crystal panel having a mosaic color filter simply is magnified and projected onto a screen. However, since this system obtains light with a desired color by absorbing light with an unwanted color out of white light from the light source by means of the color filter serving as a color selection member, only one-third or less of the white light that has entered the liquid crystal panel is transmitted (or reflected). Accordingly, the efficiency of light utilization is low and high-brightness images cannot be obtained easily. When the light source is brightened, the brightness of the displayed image can be improved. However, there remain problems of heat generation and light resistance owing to light absorption by the color filter, making it very difficult to increase the brightness.
A single-plate system that improves the efficiency of light utilization is suggested in JP 4(1992)-316296 A. FIG. 8 shows a schematic configuration of this image display apparatus.
A white light beam emitted from a light source portion 100 is led to a color separation optical system 101. As shown in FIG. 9, the color separation optical system 101 includes dichroic mirrors 121a and 121b and two reflection mirrors 121c and 121d. The dichroic mirror 121a reflects blue light and transmits green light and red light. The dichroic mirror 121b reflects red light and transmits green light and blue light. These dichroic mirrors 121a and 121b are crossed. A blue light beam 132 out of a white light beam 131 from the light source portion 100 is reflected by the dichroic mirror 121a, reflected by the reflection mirror 121d and passes through an aperture 102B of a field stop 102. A red light beam 133 is reflected by the dichroic mirror 121b, reflected by the reflection mirror 121c and passes through an aperture 102R of the field stop 102. A green light beam 134 is transmitted by both the dichroic mirrors 121a and 121b and passes through an aperture 102G of the field stop 102. The apertures 102R, 102G and 102B of the field stop 102 are formed like a belt (a rectangle), and the light beams of red, green and blue are emitted adjacent to each other from these apertures.
As shown in FIG. 8, the belt-like light beams of respective colors emitted from the field stop 102 pass through a scanning optical system 105, then illuminate different regions of a single transmission-type light valve (a display panel) 104 in a belt-like manner. With an effect of a rotating prism 103 constituting the scanning optical system 105, the belt-like light beams of red, green and blue scan the light valve 104 from the bottom to the top. When a belt-like illuminated region of one of the light beams goes beyond the uppermost end of an effective region of the light valve 104, the belt-like illuminated region of this light beam appears at the lowermost end of the effective region of the light valve 104 again. In this manner, the light beams of red, green and blue can scan over the entire effective region of the light valve 104 continuously. A light beam illuminating each row on the light valve 104 varies moment by moment, and a light valve driving device (not shown in this figure) drives each pixel by an information signal according to the color of the light beam that is illuminated. This means that each row of the light valve 104 is driven three times for every field of a video signal to be displayed. A driving signal inputted to each row is a color signal corresponding to the light beam illuminating this row among signals of the image to be displayed. The light beams of these colors that have been modulated by the light valve 104 are magnified and projected onto a screen (not shown in this figure) by a projection lens 106.
With the above configuration, the light beam from the white light source is separated into light beams of three primary colors, so that the light from the light source can be used with substantially no loss and the efficiency of light utilization can be increased. Also, since each of the pixels on the light valve displays red, green and blue sequentially, a convergence adjusting mechanism between the liquid crystal panels as in the three-plate system is not necessary, and therefore, it is possible to provide a high quality image.
However, in the above configuration, the light beams of these colors from the field stop 102 are not converged when transmitted by the rotating prism 103. Since the size (the radius of gyration) of the rotating prism 103 has to be in accordance with a region illuminated by the light beam emitted from the field stop 102, the rotating prism 103 becomes large and heavy. This has made it difficult to reduce the size and weight of the apparatus. Furthermore, a powerful motor for rotating the rotating prism 103 becomes necessary, causing an increase in the size and cost of the apparatus.
Moreover, with the above-described configuration of the color separation optical system 101, the lengths of optical paths of the light beams of individual colors from the light source portion 100 to the light valve 104 are not equal. Thus, it is impossible to focus all the light beams at a pupil position of the projection lens 106. As a result, the light quantity of the light beam focused at the pupil position and the light quantity of the light beam focused at a position shifted from the pupil position are different on the screen, resulting in poor color uniformity in the displayed image.
It is an object of the present invention to solve the above-described problems of the conventional image display apparatus and to provide a small projection-type image display apparatus that is provided with a scanning optical system for scanning an illuminated portion (a light valve) sequentially with light beams of individual colors and has enhanced color uniformity in a displayed image.
In order to achieve the above-mentioned object, a projection-type image display apparatus of the present invention includes a light source portion for emitting a white light beam; a first optical system, which includes a white illumination optical system that the white light beam from the light source portion enters and that emits a uniform white illumination light beam having a rectangular cross-section, a color separation optical system for separating the white illumination light beam into respective light beams of red, green and blue, and a relay lens system that the respective light beams obtained by a color separation enter; a rotating polygon mirror that the respective light beams having left the relay lens system enter and that scans the respective light beams while reflecting the respective light beams; a second optical system for leading the respective light beams reflected by the rotating polygon mirror to an illumination position; an image display panel that is arranged at the illumination position and provided with many pixels for modulating an incident light according to a color signal of red, green or blue; an image display panel driving circuit for driving each of the pixels of the image display panel by a signal corresponding to a color of light entering this pixel; and a projection optical system for magnifying and projecting an image of the image display panel. Here, the color separation optical system includes first and second red-reflecting mirrors that reflect at least the red light beam, first and second green-reflecting mirrors that reflect at least the green light beam, and first and second blue-reflecting mirrors that reflect at least the blue light beam. The mirrors are arranged so that optical paths of the respective light beams have equal lengths from the light source portion to the rotating polygon mirror.